While gas thermometry follows a physical law nicely, in practice it isn't all that precise.
Radars are like that too. On a very large radar proposed some years ago, we kept the areas with the RF electronics stable to one degree C, thus stabilizing phase shifts all around, despite local temperature variations due to various power components..
And even the cheap ones can be pretty good.
For the record, those "cardinal points" are generally the triple points of various pure substances.
.
formatting link
This is something that one can do at home, for some of the listed substances.
Joe Gwinn